One widely used analog to digital converter (ADC) utilizes what is known as a dual slope technique. In this technique, an operational amplifier utilizing bipolar transistors having a capacitor connected in a feedback path integrates an input signal. An input signal in the form of a standardized reference current is fed to a current summing junction through an analog gate. The standardized reference current is of opposite polarity to an analog input current which input current is applied to the current summing junction. The combined analog input current and the gated standardized reference current at the current summing junction is applied to the input of the integrator. The integrated output is applied to the analog gate through suitable circuit means to actuate the gate to a closed condition at a predetermined level of the output from the integrator. A time based oscillator is connected to the analog gate for actuation of the gate to an open condition at predetermined time intervals. The ratio of the time during which the analog gate is open to the time of the complete operating cycle during which the gate is both opened and closed is a measure of the analog input current to the converter. In essence, a time ratio function is developed expressly for the analog input. The time ratio function is then applied to switch a time based oscillator scaled to provide a digital output. The digital output represents the amplitude of the analog signal.
Many forms of this so-called dual slope ADC have been developed over the past few years. However, each of the systems have required mainly an integrator of the Miller or similar type utilizing an operational amplifier and capacitor in the feedback loop. This type of integrator conventionally utilizes linear active devices such as bipolar transistors. However, the timing function including means for providing the clock signals and for providing the digital output from the converter may be digital devices. As a result, the usual configuration for the present state of the art ADC's are two so-called "chips", wherein one chip packages the bipolar linear devices and the other chip packages the digital components. This system is both costly and complicated to fabricate in that the interconnection of two separate chips is required.